.first {
    /* 为了让子孙后代可以参照自己进行绝对定位 */
    position: relative;
    width: 80vw;
    height: 50px;
    border: 1px solid blue;
    margin: auto;
}

.progress {
    /* 结合上下文可以知道，这里是相对于最近的已定位父元素进行绝对定位 */
    position: absolute;
    left: 0;
    top: 0;
    z-index: 1;
    height: 50px;
    width: 25%;
    background: #9696ff;
}

.percent {
    position: absolute;
    left: 0;
    top: 0;
    z-index: 2;
    width: 100%;
    height: 50px;
    line-height: 50px;
    text-align: center;
    user-select: none;
}

.second {
    width: 80vw;
    border: 1px solid blue;
    margin: 100px auto;
    user-select: none;
    cursor: pointer;
}

.second ul { list-style: none; margin: 0; padding: 0; }
.second>ul { height: 30px; }
.second li.menu {
    position: relative;
    float: left;
    width: 50%;
    height: 30px;
    line-height: 30px;
}

.second .menu>ul {
    display: none;
    position: absolute;
    left: 0;
    /* top: -90px; */
    bottom: 30px;
    width: 100%;
    background: #dfdfdf;
}

.second .menu:hover>ul {
    display: block;
}

.second .menu:hover>ul li:hover {
    background: #ff0;
}

.third {
    width: 1226px;
    height: 460px;
    margin: 25px auto;
    background: url("redmi-k50.jpg") center/cover ;
}

.category-list {
    /* 采用绝对定位或相对定位且z-index取值不为auto的元素属于堆叠上下文 */
    position: relative;
    z-index: 0;

    width: 234px;
    height: 459px;
    border-top: 1px solid transparent;
    background: rgba(180, 180, 180, 0.75);
}

.category-list>ul {
    padding: 0;
    margin: 19px 0;
    list-style: none;
}

.category>a {
    display: flex;
    flex-flow: row nowrap;
    justify-content: space-around;
    height: 3em;
    font: 14px/3em "微软雅黑";
    color: #fff;
    cursor: pointer;
}

.category>a:hover {
    background: #ff6700;
}

.category>a span:first-child {
    flex-basis: 60%;
}

.category>a span:last-child {
    flex-basis: 10%;
    text-align: right;
}

.subcategory {
    display: none;
    position: absolute;
    top: 0;
    left: 234px;
    width: 990px;
    height: 458px;
    flex-flow: column wrap;
    justify-content: space-around;
    align-content: space-between;
    border: 1px solid #dfdfdf;
    box-shadow: 3px 3px 30px -10px #dfdfdf;
    background: #fff;
}

/* 当鼠标悬浮到`.category`时就将`.subcategory`设置为弹性容器 */
.category:hover>.subcategory {
    display: flex;
}

.subcategory>* {
    /* 
      当鼠标悬浮到`.category`时`.subcategory`就是弹性容器，
      作为弹性容器的直接子元素(即弹性项目)若`z-index`取值不为`auto`则会形成堆叠上下文
    */
    z-index: 1;
    flex-basis: 40px;
    width: 234px;
    box-shadow: 0 0 5px 4px blue inset;
}